The readout already grouped usage by source — `group_by(event, source)` —
and the loop directly below it threw the source away, collapsing every
surface into one corpus-wide ratio. So the question a threshold is
actually tuned against, "is THIS surface worth its noise", could not be
asked of any surface, while the data to answer it sat in the table.
`usage.by_source` reports notes_surfaced / notes_pulled / pull_through
per surface. The grain is the note, not the call: a pull records the
door it came through, not the surface that led there, so grouping the
pulled rows by source would answer a different question. Joining
surfaced rows to pulled rows on note_id answers this one without the
session identity #2085 declined to invent — at the cost of being an
upper bound per surface, which the docstring says where it is read.
Ambient surfaces report counts and a null ratio: nothing chose those
records, so "surfaced often, opened never" is not a judgment about them.
A surface that genuinely produced nothing reports 0.0, which must not
look like the null.
The join is guarded separately from the two reads above it. #2663 was a
novel SQL shape the database rejected inside a broad except; this is the
novel shape here, and it must not take down two readouts that work.
Tests are integration for that same reason — a mock passes on a query
Postgres refuses. They pin the distinct-first property (three surfacings
of one note are one note), the ambient null, and the LIKE escape, since
an unescaped `mcp_%` also matches `mcpXget_note` and nothing else in the
payload would show the difference.
bvandeusen
0d4b155699